1. Field of the Invention
This invention relates to a method of forming bonding metal bumps for fixing an optical device array on the surface of a submount which is employed in a parallel transmission optical module or the like used for optical communication purpose.
2. Description of the Prior Art
Recently, applications for optical communications have been increasingly expanding so that optical elements such as optical fiber, laser diode (LD), light emitting diode (LED), photo-diode (PD), optical switch, optical modulator, optical isolator, optical waveguide and so on are made increasingly functional. In order to make the transmission of larger amounts of information possible, a method has been proposed so that the data transmission can be carried out in parallel and in real time between computer terminals, exchangers and/or large-scale computers. As the device to be used for the purpose of parallel transmission of information, a parallel transmission optical module where a plurality of light emitters and/or a plurality of light receivers are used integrally with a plurality of optical fibers is known.
The parallel transmission optical module generally uses, as the light emitter, a LED/LD array having a plurality of either LEDs or LDs arranged in a monolithic manner on a semiconductor substrate. On the other hand, it uses, as the light receiver, a PD array which has a plurality of PDs arranged thereon in the same manner as above. On the back surface of each of these optical device arrays, a plurality of electrodes made of gold (Au) or the like are provided for electrical connection or positioning purposes. The optical device arrays thus prepared are mounted on a submount which becomes an interface with an external electric circuit. The submount is made of alumina (Al.sub.2 O.sub.3), aluminum nitride (AlN) or the like, and on one side surface of which are patterned a plurality of layers with chromium (Cr)/nickel (Ni), chromium (Cr)/gold (Au) or the like by the thin film forming process thereby to provide an electric interconnection having a plurality of layers disposed in a laminated pattern. At the connecting portions of the electric interconnection with the optical device arrays, electrodes are patterned thereby to connect the optical device arrays via the electrodes to the submount.
In case of mounting the optical device array on the submount, it may be warped in general due to the difference in layer structure between the front surface and back surface of the array itself. In order for electrodes, each with a thickness of 2 to 3 .mu.m, provided on the array and the electrodes, each with a thickness of 2 to 3 .mu.m, formed on the submount to be in perfect contact with each other, a pressure is required to be applied. In this case, however, the arrays are formed on the substrate made of such fragile materials as gallium arsenide (GaAs), indium phosphide (InP) and so on. Thus the application of a pressure may cause them to be damaged. In order to overcome such a problem, conventionally, bonding metal bumps each having a height of about 20 to 30 .mu.m are provided at the connecting portions of either the submount or the array. This allows the warp of the array to be absorbed so that connection can be provided via the bumps without applying a pressure. The bonding metal to be used for this purpose, for example, lead (Pb) - tin (Sn) alloy, gold (Au) - tin (Sn) alloy and so on is known. In connecting such members as shown above by the reflow bonding technique, the bonding metal bumps exert the self-alignment effect themselves by surface tension, so that these members can be positioned very accurately.
In addition, a dam made of polyimide resin or the like is provided in advance so as to surround each of the connecting portions of them thereby to form a cavity. The bonding metal bumps are respectively arranged into the cavities thus formed, so that the bonding metal bumps can be prevented from being drained out therefrom when melted by a heating.
In the case of the parallel transmission optical module, the formation of bonding metal bumps on the submount can be easily achieved by using a Pb-Sn alloy as the bonding metal and arranging it onto each of the electrodes formed on the surface of the submount by plating process. This method, there generates a slight variation per hour (about 1 .mu.m) at the portions provided on the array to be connected with the electrodes. Thus, it is not preferable to be used for connecting a highly reliable optical device requiring highly accurate positioning on a long term basis.
Therefore, Au-Sn alloy which does not generate such variation per hour has been used conventionally. In this case, however, the mixing ratio of Au and Sn is required to be controlled precisely, so that the plating technique is difficult to apply. As a result, generally, the Au-Sn alloy is deposited onto the electrodes in a film form using a vacuum deposition technique. However, many processes are unavoidably required to form a film with a thickness of several tens of micrometers by the vacuum deposition technique, decreasing productivity.
A method that the Au-Sn alloy is arranged in the ball form onto each of the electrodes formed on the submount will be considered. However, the arrangement of small balls each about 100 .mu.m in diameter one by one on each of the electrodes is extremely sophisticated, and thus not practical. So another method will be considered in which a large number of balls made of Au-Sn alloy are scatteredly provided over the submount. Then, unnecessary balls not arranged on the electrodes are removed by shaking. Necessary balls arranged on each of the electrodes remain.
However, this method may result in a fault in connection caused by some balls remaining while not arranged on the electrodes.
Accordingly, an object of this invention is to provide a method of easily and precisely forming bonding metal bumps on a submount for use with an optical device array.
Another object of this invention is to provide a method of forming bonding metal bumps on a submount for use with an optical device array, which is extremely high in productivity and low in cost.